In a variety of contexts, it is desirable to obtain dimension related information for a flow channel where it is difficult or undesirable to access the flow channel or otherwise obtain a direct dimensional measurement. Obtaining such information is particularly challenging where channel dimensions vary over time. An example is obtaining dimension related information for a blood vessel (i.e., artery or vein), such as the ascending aorta, of a human or other patient. The dimension related information of interest maybe a dimension of the flow channel or other information derived from or dependent on dimension such as quantitative flow rate information (e.g., volumetric or mass flow rate), vessel elasticity/health, volumetric delivery per heartbeat, ejection fraction or the like.
A channel dimension related characteristic of particular interest is volumetric flow rate (“VFR”). The volumetric flow rate (“VFR”) of a fluid flowing through a channel is dependent on the velocity of the fluid and the cross-sectional area of the channel. Instantaneous VFRs may be calculated when these values are known between short time intervals that may be considered instantaneous. Determining the VFR of fluids in patients is useful, for example, in assessing cardiac performance. In this application, the fluid is a liquid, e.g., blood, that is flowing through a closed channel, such as the aorta. Unfortunately, variability between individual patients prevents knowing the dimensions of a channel inside the patient absent a measurement to determine the channel dimensions.
Presently, the principle technologies used to measure cardiac blood flow employ time-varying markers, such as a dye, chilled normal saline, or blood warmed with a small electric heater, introduced into the heart through a catheter. The rate of change of the marker is then used to estimate the overall flow rate of blood. The catheter is most commonly introduced into the patient's femoral artery and threaded through the venous system into and through the heart to the pulmonary artery. A variant of this method introduces a dye without a catheter, but requires an injection near the atrium and measures the dye concentration at a point such as the ear. Unfortunately, these time-varying marker methods rely on the marker dilution transient and provide bulk flow data, rather than flow rate data through a channel. In addition, the time-varying marker methods do not provide instantaneous VFRs.
Another group of methods for measuring cardiac flow, known as “Fick methods,” calculate cardiac output using the difference in oxygen content between the arterial and mixed venous blood and the total body oxygen consumption. The classic Fick method uses arterial and pulmonary artery catheters to measure the oxygen content. Related methods measure oxygen and carbon dioxide data in a patient's airways to avoid using catheters. However, similar to the time-varying marker methods, Fick methods measure bulk cardiac output, and not flow rates. Finally, other methods, such as bioimpedance, may also be used to measure cardiac output. However, as with the Fick methods and time-varying marker methods, these methods do not measure flow rate, but rather only cardiac output.
Certain methods exist for non-invasively determining fluid velocities, with Doppler methods being the most common in the context of blood flow analysis. Such methods, however, only measure fluid velocity and require another measurement to provide the channel dimensions if quantitative flow rate information is desired. Such channel dimension measurements have sometimes been obtained based on methods related to imaging or time delays. X-rays and radiopaque dyes may also be used, but the results are time consuming and difficult to interpret. Further, the time-delay of echos from an ultrasound signal may be used to provide an estimate of the channel dimensions, but obtaining precise results has required disposing the ultrasonic transducers in close proximity to the channel, something that is not possible unless the devices are introduced into the patient. Therefore, significant deficiencies exist in relation to non-invasive determination of a VFR of fluid in a channel.
Conventional flow channel measurements may also be problematic, uncertain or inaccurate due to measurement artifact and associated processing. For example, ultrasound measurements of an ascending aorta are problematic due to the difficulty of isolating the signal of interest from measurement artifact. Such artifact may relate to echos from physiological material outside of the aorta or other channel that tend to obscure the signal of interest, in-channel noise sources and other interfering components. Some approaches have attempted to minimize certain artifact components by disposing the probe in close proximity to the aorta, thus incurring a morbidity trade-off. Other approaches attempt to minimize certain artifact components by employing a very narrow signal, but thereby complicate targeting of the aorta or other channel under consideration.